For a GNSS (e.g. GPS) receiver, sensitivity is a major performance criterion. TTFF (Time to First Fix) is a representative standard for receiver sensitivity. To speed up TTFF, a technique called AGPS (Assisted Global Positioning System) is developed to promote TTFF performance. In an AGPS system, an assistant information is provided to a remote receiver so that the receiver can fix positions of satellites in a shortened period of time. An important portion of the assistant information is satellite navigation message, such as ephemeris, or satellite trajectory prediction data. The orbit determination technology and satellite trajectory prediction can be implemented by using a plenty of ranging observations from reference ground network stations, which can be simply referred to as ground observations. Practically, the processing of ground observations is complicated, thus strong calculation capability is required to execute the processing. As it is known, there must be some prediction errors existing in satellite trajectory extension, i.e. satellite trajectory predication, due to imperfect satellite trajectory prediction models. Therefore, prediction of the satellite trajectory cannot be extended unlimitedly. Currently, it is possible to predict the satellite trajectory for the up-coming 7 to 14 days.
For the reason mentioned above, only a device with high calculation capability such as a server is sufficient to support the satellite trajectory prediction. The server calculates the predicted satellite trajectory and passes the calculated satellite trajectory or equivalent data set to an AGPS server. Then the AGPS server provides the satellite trajectory prediction or equivalent data set to users through connection. According to the conventional schemes, it is difficult to execute the satellite trajectory prediction on a mobile apparatus such as a PDA (Personal Digital Assistant), a smart phone, a GPS apparatus or the like. Therefore, it is an object of the present invention to provide a method and a device for using GNSS satellite trajectory prediction that can be implemented and resided on a host processor, which can be embedded in a mobile apparatus.
FIG. 10 is a flow chart showing a conventional process of obtaining and using acquisition assistance (AA) data to acquire satellite signals. As shown, estimated time (S1002), rough user position (S1004) of a remote receiver and an estimated satellite position (S1006) are computed based on satellite trajectory prediction such as long term orbit data at a server or obtained from the broadcast ephemeris, which is valid in two hours in general. The remote receiver then computes the AA data according to the position and time information in step S1010. For example, the remote receiver predicts possible Doppler shifts for the satellite in view of the remote receiver. That is, the AA data at least comprises predicted Doppler shifts for the satellite in view of the remote receiver. In GPS, each satellite has its satellite signal leave the satellite at a frequency of 1575.42 MHz. As known, the frequency of the satellite signal observed at the remote receiver will be shifted about ±4.5 KHz due to relative satellite motion. This is known as a Doppler shift. Then, a window of uncertainly range s provided for the satellite based on the AA data in step S1020. This window defines a certain range of the Doppler shift and a range of code phase (i.e. certain code chips of the satellite signal) for the satellite. The size of the window is determined depending on the accuracy of the estimated position and time. In step S1030, the remote receiver acquires the satellite signals using the AA data within the range of the window so as to avoid from searching the satellite in the widest range (i.e. all the Doppler shifts and all the code phases), which is referred to as “open sky searching”. In step S1040, a pseudorange, which is the time for the satellite signal flying from the satellite to the remote receiver multiplied by the speed of light, in time domain and a Doppler shift in frequency domain are computed from the acquired satellite signal. In step S1050, it is determined whether the number of the acquired satellites has been exceeded four. As well known in this field, to fix a position, at least four satellites must be required. If so, then the user position (i.e. the position of the remote receiver) can be fixed in step S1060. Otherwise, the remote receiver keeps searching (step S1070), and the process goes back to step S1020.
Under a circumstance where the valid ephemeris is not available, the remote receiver needs the assistance of the server in AGPS. The server provides the necessary data about the position and time to the receiver so that the receiver can compute the AA data so as to promote the performance of TTFF. If the remote receiver fails to get in connection with the server in any manner, then the receiver cannot compute the AA data and must execute the open sky searching (ex, searching all the satellites in the open sky with all code phases and Doppler shifts.). As can be known, it takes a great period of time to do the open sky searching. This will adversely influence the TTFF.